Characterisation and electrocatalytic activity of PtNi alloys on Pt{111} electrodes formed using different thermal treatments

The preparation of two different types of PtNi overlayer supported on Pt(1 1 1) together with their characterisation by cyclic voltammetry (CV), scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) is reported. The first PtNi surface was created by resistively heating to approximately 1100 K a 10-2 M droplet of aqueous nickel (II) chloride solution attached to a Pt{1 1 1} Clavilier bead electrode in the nitrogen/water atmosphere of the electrochemical cell. From STM, it was evident that the crystal surface was covered with small, closely packed and uniform islands visible over the whole surface. XPS data indicated that although platinum was still present in its metallic state, the surface of the islands consisted largely of a Ni oxide/hydroxide phase. Subsequent electrocatalytic analysis showed that this oxidised PtNi surface exhibited lower activity than pure platinum towards the oxygen reduction reaction (ORR). The second PtNi surface was formed by subsequently flame annealing the Ni oxide/hydroxide adlayer to approximately 900 K outside of the electrochemical cell and cooling the electrode rapidly in a stream of hydrogen. STM showed that again, small islands had formed which, according to XPS, corresponded to a metallic PtNi phase. However, the surface produced was flatter than before with clear terrace structures also being observed. This behaviour overall would be consistent with preferential segregation of either Pt or Ni (oxide/hydroxide) from a PtNi alloy depending on the oxidising nature of the ambient after annealing and cooling as reported previously by Kibler for PtRu alloys. By controlling the number of flame annealing treatments, CV revealed a gradual diminution in the amount of nickel present at the selvedge. The reduced, hydrogen cooled platinum-rich PtNi phase exhibited a current density for ORR ten times higher than that of Pt{1 1 1} (as measured by the current density in mA/cm(2) at 0.9 V) and a positive shift in the value of E1/2 by approximately 70 mV. (C) 2013 Elsevier B.V. All rights reserved.